1. Field of the Invention
The present invention relates to a thin film semiconductor device and a method for forming the same. More particularly, the present invention relates to a thin film semiconductor device and a method for forming the same which excel in confidence, productivity, and yield thereof. The present invention is applicable to e.g. a three dimensional integrated circuit or a driving circuit used in a liquid crystal display or a thin film image sensor and the like.
2. Description of the Related Art
Heretofore, a semiconductor integrated circuit has been mainly a monolithic type, which has been formed on such semiconductor substrate as silicon. Recently, it has been tried to form it on an insulating substrate made of glass, sapphire, and the like. This is caused by the reasons that an operation speed will be raised according to the lowering of parasitic capacity between a substrate and a wiring, especially quartz or glass materials are not limited in size like a silicon wafer, cheap, easy to separate each device, and further a latch-up phenomenon, which poses a problem in a monolithic circuit of CMOS, will not arise. Also, it is necessary to form a thin film transistor (hereinafter refer to as TFT) etc. on a transparent substrate, because a semiconductor device and a liquid crystal element, or a light detecting element are required to be composed in a body, in a liquid crystal display and a contact type image sensor.
According to these reasons, a thin film semiconductor device has come to be formed on an insulating substrate. As shown in FIG. 5 which represents an instance of the usual thin film semiconductor device, a film 503 of silicon oxide etc. is formed on an insulating substrate 501, as a passivation film, and thereon, TFT is formed independent of other TFT. TFT has a source (drain) region 507, a drain (source) region 509, a channel forming region 508 (simply called as a channel region) which is held between the said regions of 507 and 509, a gate insulating film 504, a gate electrode 510, a source (drain) electrode 511, and a drain (source) electrode 512, just like MOSFET of a monolithic integrated circuit. Also, there is provided with an interlayer insulation 506 of PSG (phosphosilicate glass) so that a multilayer interconnect can be made.
The instance of FIG. 5 is called as a coplanar type, but, TFT has further different types such as an inverse coplanar type, a stagger type, and an inverse stagger type, according to the arrangement of a gate electrode and a channel region. These types are well known in the usual literatures to which the details can be referred.
Also in case of the monolithic integrated circuit, such alkaline ion as natrium and potassium, or such transition metal ion as iron, copper, and nickel poses a serious contamination problem, and much attention has been paid to stop the intrusion of these ions. In case of TFT, the problem of these ions are important alike, and close attention has been so given to the purification in a manufacturing process as to avoid the ion contamination to the utmost. And also a countermeasure has been taken so that the contamination of these ions do not extend to the device.
A thin film semiconductor device is different from a monolithic integrated circuit, in a point that a contaminated ion concentration in a substrate of the former is rather higher than that of the latter. That is, a single silicon crystal used in a monolithic integrated circuit has been so manufactured as to eliminate these harmful elements, based on the long experienced technologies. The content of these elements contaminated in the present products on sale is 1010 cm−3 or less.
However, in general, the concentration of elements contaminated in an insulating substrate usable for a thin film semiconductor device is not low. Of course, it is theoretically possible to reduce a strange element concentration, which will be the above contamination source, in case of such single crystal substrate as a spinel or a sapphire substrate. But it is not actual from a profitable point of view.
Also, in case of a quartz substrate, it is ideally possible to suppress the intrusion of the strange element, if it is prepared in a gaseous reaction (vapor reaction), using a high purity of silane gas and oxygen as a raw material. But it is difficult to send out the strange element to the outside in case it is once taken in, as the structure of quartz substrate is amorphous. A substrate used in a liquid crystal display gives a priority to the cost problem in particular, and then it is needed to use a cheap one, which originally contains various sorts of strange elements so as to make the manufacturing and the processing easy. There are strange elements which are not desirable in themselves for a semiconductor device, and there are cases where the strange elements will be mixed from the outside in the process of their adding, or these are contained in an adding material as an impurity.
Since for example, TN glass is a cheap glass substrate, heat resisting, and equivalent to silicon in a heat expansion coefficient, it is desirable for a substrate used in a liquid crystal display. But it contains about 5% of lithium, a portion of which is ionized to intrude into a semiconductor device as a mobile ion, and to deteriorate a device grade. Also it is difficult to prepare a high purity lithium of 99% or more, and about 0.7% of natrium is usually contained therein. An ionization ratio of natrium is very high to show about 10%, which brings about a very serious effect on the device features.
As shown in FIG. 5, in the conventional thin film semiconductor device, silicon oxide etc. has been used as a passivation film to cope with the intrusion of these mobile ions. Also these ions have been dealt with by gettering them, employing PSG (Phosphosilicate glass) or BPSG (Borophosphosilicate glass) as a interlayer insulation. However, it has been difficult to prevent the contamination effectively by these methods.